In recent years, global scale environmental destruction problems such as global warming, ozone layer depletion, acid rain, etc. have gained prominent attention. Internationally, environmental pollution measures are advocated, and various regulations have been established from the viewpoint of environmental protection. Among them, release into the atmosphere of organic solvents (VOC) has been a serious problem, and also in each industry, the movement for de-organic solvents (de-VOC) along with the trend for strengthening VOC regulations has become active. Also in the paint industry, as a substitute for conventional organic solvent-based coating material, a powder coating material is highly expected as a coating material which contains no VOC and which is friendly to the environment as it can be recovered and reused without requiring exhaust treatment or wastewater treatment.
As such a powder coating material, an acrylic resin powder coating material, a polyester resin powder coating material or an epoxy resin powder coating material, is mainly used. However, cured films formed by using these powder coating materials have such a drawback that they are poor in weather resistance.
As a powder coating material to overcome such a drawback, a fluororesin type powder coating material using a fluororesin has been developed (see e.g. Patent Document 1).
Further, for the purpose of e.g. cost reduction, a powder coating material having a polyester resin and a fluororesin blended, has been proposed (see e.g. Patent Document 2).
The powder coating material having a polyester resin and a fluororesin blended, is excellent in weather resistance of the cured film, as compared with an acrylic resin-type powder coating material, a polyester resin-type powder coating material and an epoxy resin-type powder coating material. However, when the cured film contains titanium oxide as a pigment, the weather resistance of the cured film may become insufficient.
Further, it is possible to form a cured film excellent in weather resistance with low cost by carrying out so called two-coatings which comprise coating the surface of a substrate with a polyester resin-type powder coating material to form a polyester layer and coating the surface of the polyester layer with a fluororesin-type powder coating material. However, in such two coatings, it becomes cumbersome to form the cured film.